In recent years there has come into use an arrangement in which a counter is provided on the side of a machine tool for counting output detection signals from a position sensor and the current position (the coordinate value) of a movable machine part (a motor, for example) is obtained on the basis of the count value of the counter on the side of a numerical controller. The provision of such a counter on the side of the machine tool precludes the necessity of performing a zero return upon each turning-ON of the power source on the side of the numerical controller, but involves the necessity of providing a signal line for transmitting the count value of the counter to the numerical controller, posing a problem of increasing the number of signal lines between the machine tool and the numerical controller.
As a solution to this problem there has been proposed an arrangement in which a switching circuit actuable for selecting the detection signal of the position sensor and the count value of the counter is provided in the machine tool and the output of the switching circuit is transmitted to the numerical controller. With the provision of such a switching circuit, it is possible to transfer the detection signal of the position sensor and the count value of the counter through one signal line. However, when the machine tool is activated by some cause during the transfer of the count value of the counter to the numerical controller, there will be an error between the actual current position of the movable machine part and the current position of the movable machine part recognized on the side of the numerical controller.